1. Field of the Invention
This invention relates to a crankcase arrangement for an engine, and more particularly relates to a crankshaft bearing structure suitable for a vertically oriented engine.
2. Description of Related Art
A typical outboard motor employs an internal combustion engine disposed within a power head of the motor. The engine includes a crankshaft that extends generally vertically. The crankshaft is confined in a crankcase cavity defined by a cylinder body and a crankcase member, and is rotatably journaled by a bearing structure formed between the cylinder body and the crankcase member. The bearing structure thus extends generally transversely across an axis of the crankshaft in the crankcase cavity.
The engine normally includes a lubrication system to lubricate various moving portions of the engine. The crankshaft bearing structure is one of such portions that need lubrication. Lubricant (e.g., oil) often is supplied to the crankshaft bearing structure from a lubricant reservoir by a lubricant pump through one or more lubricant passages formed within the cylinder body. The lubricant reservoir is normally disposed in a driveshaft housing of the outboard motor, which depends from a power head, and the crankshaft usually drives the lubricant pump.
The oil lubricates the crankshaft bearing structure and then is spattered toward an internal side wall of the crankcase member by the centrifugal force produced with rotation of the crankshaft. The majority of the lubricant first adheres on the wall and then cascades down to the bottom of the crankcase cavity so as to return to the lubricant reservoir under gravity.
The flow direction of the lubricant, however, is across the transversely extending crankshaft bearing structure. This construction of the bearing structures, which protrudes from the sidewall of the crankcase member, blocks the lubricant from flowing downwardly to the lubricant reservoir.
In one prior arrangement, such as that disclosed by Japanese Laid-Open Patent Publication No. Hei 09-273406, through-holes are provided in the respective bearing sections adjacent to the sidewall of the crankcase member so as to permit the lubricant to fall more freely towards the bottom of the crankcase. Japanese Laid-Open Patent Publication No. Hei 09-273407 discloses another arrangement to address the foregoing problem. In this arrangement, a bearing cap is provided separately from a crankcase member and a lubricant return passage is defined between the bearing cap and the crankcase member. Both constructions, however, suffer from other disadvantages.
If, in the arrangement of the former publication, the through-holes were large enough to produce a generally free flow of the lubricant, the rigidity of the bearing sections would be significantly reduced and the bearing structure would be weakened. Such Large holes in the slender section of the bearing section would weaken these structures. Conversely, smaller holes would not permit the lubricant to return quickly to the lubricant reservoir. The quick return of the lubricant to a lubricant reservoir is required because the lubricant must be recycled through the engine.
In the latter arrangement, the uppermost and lowermost bearing sections are slender. While this construction affords some clearance between the bearing cap and the crankcase member without increasing the height of the engine, it also weakens the bearing upper and lower bearing structure.
A need therefore exists for an improved crankshaft bearing structure for an engine that can allow the lubricant to return quickly to the lubricant reservoir from the crankcase even though the engine has a compact structure.